Takahashi Kazunori, Charles Christine, Boswell Rod W
Department of Electrical Engineering, Tohoku University, Sendai, 980-8579, Japan.
Interdisciplinary Research Center for Non-equilibrium Plasma, Tohoku University, Sendai, 980-8579, Japan.
Sci Rep. 2022 Dec 5;12(1):20137. doi: 10.1038/s41598-022-24202-9.
Plasma flows in divergent magnetic fields resembling a magnetic nozzle can be found over wide scales ranging from astrophysical objects to terrestrial plasma devices. Plasma detachment from a magnetic nozzle is a frequent occurrence in natural plasmas, e.g., plasma ejection from the Sun and release from the Sun's magnetic field, forming the solar wind. Plasma detachment has also been a challenging problem relating to space propulsion devices utilizing a magnetic nozzle, especially the detachment of the magnetized electrons having a gyro-radius smaller than the system's scale is required to maintain zero net current exhausted from the system. Here we experimentally demonstrate that a cross-field transport of the electrons toward the main nozzle axis, which contributes to neutralizing the ions detached from the nozzle, is induced by the spontaneously excited magnetosonic wave having the frequency considerably higher than the ion cyclotron frequency and close to the lower hybrid frequency, driving an E × B drift that only effects the electrons. Wave-induced transport and loss have been one of many important issues in plasma physics over the past several decades. Conversely, the presently observed electron inward transport has a beneficial effect on the detachment by reducing the divergence of the expanding plasma beam; this finding will open a new perspective for the role of waves and instabilities in plasmas.
从天体物理对象到地面等离子体装置，在广泛的尺度范围内都能发现等离子体在类似磁喷嘴的发散磁场中流动。在自然等离子体中，例如从太阳喷射出的等离子体以及从太阳磁场释放出来形成太阳风的过程中，等离子体从磁喷嘴分离是经常发生的现象。对于利用磁喷嘴的空间推进装置而言，等离子体分离也是一个具有挑战性的问题，特别是需要使回旋半径小于系统尺度的磁化电子分离，以维持从系统排出的净电流为零。在此，我们通过实验证明，频率远高于离子回旋频率且接近低杂波频率的自发激发磁声波会引发电子向主喷嘴轴线的横向输运，这种输运有助于中和从喷嘴分离的离子，驱动仅作用于电子的E×B漂移。在过去几十年里，波致输运和损失一直是等离子体物理学中的诸多重要问题之一。相反，目前观察到的电子向内输运通过减少膨胀等离子体束的发散，对等离子体分离具有有益作用；这一发现将为波和不稳定性在等离子体中的作用开启新的视角。
